A variety of mount assemblies are presently available to isolate vehicle vibrations, such as for automobile and truck engines and transmissions. One of the most popular mounts today is the hydraulic-elastomeric mount of the type disclosed in U.S. Pat. No. 4,588,173 to Gold et al., issued May 13, 1986, entitled "Hydraulic-Elastomeric Mount" and assigned to the assignee of the present invention.
The hydraulic mount assembly of this prior invention includes a reinforced, hollow rubber body that is closed by a resilient diaphragm so as to form a cavity. This cavity is partitioned by a plate into two chambers that are in fluid communication through a relatively large central opening in the plate. The first or primary chamber is formed between the plate and the body. The secondary chamber is formed between the plate and the diaphragm.
A decoupler is positioned in the central opening of the plate and reciprocates in response to the vibrations. The decoupler movement alone accommodates small volume changes in the two chambers. When, for example, the decoupler moves in a direction toward the diaphragm, the volume of the portion of the decoupler cavity in the primary chamber increases and the volume of the portion in the secondary chamber correspondingly decreases, and vice-versa. In this way, for certain small vibratory amplitudes as occur at generally higher frequencies, fluid flow between the chambers is substantially avoided and undesirable hydraulic damping is eliminated. In effect, this freely floating decoupler is a passive tuning device.
In addition to the relatively large central opening, an orifice track with a smaller, restricted flow passage is provided, extending around the perimeter of the orifice plate. Each end of the track has an opening. One opening communicates with the primary chamber. The other opening communicates with the secondary chamber. The orifice track provides the hydraulic mount assembly with another passive tuning component, and when combined with the freely floating decoupler, provides at least three distinct dynamic operating modes. The particular operating mode is primarily determined by the flow of fluid between the two chambers.
More specifically, small amplitude vibrating input, such as from relatively smooth engine idling or the like, produces no damping due to the action of the decoupler, as explained above. Large amplitude vibrating input, such as heavy engine loading during sudden accelerations or panic stops, produces high velocity fluid flow through the orifice track, and accordingly, a high level of damping force and desirable smoothing action. A third or intermediate operational mode of the mount occurs during medium amplitude inputs experienced in normal driving and resulting in lower velocity fluid flow through the orifice track. In response to the decoupler switching from movement in one direction to another in each of the modes, a limited amount of fluid can bypass the orifice track by moving around the edges of the decoupler and through the central opening thereby smoothing the transition.
This basic mount design has proved quite successful, and represents a significant advance over the prior art engine mounts, particularly those of the solid rubber type. More specifically, hydraulic mounts provide a more favorable balance of load support and damping control. It should be appreciated, however, that additional improvement in operating characteristics is still possible.
To this end, more recent developments in hydraulic mount technology have lead to the advent of electronic control of the dynamic characteristics of the mount. Active, rather than passive, control allows more efficient and effective isolation of vibrations and suppression of noise. A previously developed hydraulic mount of the active control type is disclosed in U.S. Pat. No. 4,783,062 to Hamberg et al., issued Nov. 8, 1988, entitled "Electronic Hydraulic Mount/Internal Solenoid" and assigned to the assignee of the present invention.
In this mount assembly, the partition includes three passages connecting the primary and secondary chambers. One of the passages is a central opening, but no decoupler is specified in the preferred embodiment. Two additional passages of varying length form orifice tracks providing unique damping characteristics tuned to isolate selected frequencies of vibration and provide the desired engine control. A sliding gate extends across the entry to the central opening and the two passages. This gate is displaceable to direct the flow of fluid between the primary and secondary chambers through a selected passage or passages in the partition.
A solenoid actuator mounted on the partition includes multiple electric coils that allow the positive positioning of the gate. A control circuit with on-board transducers is provided to monitor vehicle operating and road conditions. A microprocessor acts in response to the sensed conditions causing the necessary sequential energization of the series of coils to properly position the gate and provide the desired damping characteristics.
The mount assembly described in the Hamberg et al. patent is particularly adapted for tuning to the specific resonance frequencies characteristic of the vehicle component being damped. This allows the mount assembly to more efficiently and effectively isolate vibrations and suppress noise over a wide range of vehicle operating and road conditions.
While the mount assembly disclosed in the Hamberg et al. patent may be very effectively tuned to provide the desired damping and dynamic rate characteristics over a wide range of vehicle operating conditions, still further improvements in mount assembly design are possible. More particularly, it is desirable to provide a mount assembly that relies upon passive tuning features to provide operating and performance characteristics substantially as effective as those provided by an active tuning system. Such a passive, mechanically actuated mount assembly is significantly less expensive to produce requiring neither electrical nor pneumatic control. Additionally, no sensors for monitoring operating conditions are required. Accordingly, such a system is also less complicated and more reliable.
Further, it is particularly desirable to provide a passive mount assembly that is tuned to exhibit a reduced dynamic rate over a selected range of low or small amplitude/relatively high frequency engine vibrations. More particularly, present state of the art mount assemblies, either passive or active, do not fully compensate for the change in the flow characteristics of the hydraulic fluid that is believed to take place at higher frequencies; i.e. the fluid transitions from laminar to turbulent flow causing a change in expected operational characteristics. As a result of the turbulent flow, both the decoupler passage and orifice track(s) become restricted, eventually becoming effectively choked off. This prevents continued fluid flow between the chambers that is critical for proper damping and vibration/noise control.
The flow cut-off results in a significant pressure buildup in the primary chamber of the mount that causes a very sharp increase in the dynamic rate characteristics. The resulting increase in stiffness caused by the high dynamic rate prevents the best suppression and isolation of low amplitude/relatively high frequency vibrations. A need is therefore identified for a mount assembly providing improved tuning of the higher frequencies; that is, in the range of 10-200, and particularly in the low-to-medium part of the range.